This invention relates to a nonaqueous electrolyte for a primary electrochemical cell, such as a lithium/iron disulfide cell. More specifically, a ternary electrolyte including dioxolane, dimethoxyethane and a linear ether with asymmetric end groups is contemplated.
Batteries are used to provide power to many portable electronic devices. In today's consumer-driven device market, standardized primary cell sizes (e.g., AA or AAA) and specific nominal voltages (typically 1.5 V) are preferred. Moreover, consumers frequently opt to use primary batteries for their low cost, convenience, reliability and sustained shelf life as compared to comparable, currently available rechargeable (i.e., secondary) batteries. Primary lithium batteries (those that contain metallic lithium or lithium alloy as the electrochemically active material of the negative electrode) are becoming increasingly popular as the battery of choice for new devices because of trends in those devices toward smaller size and higher power.
One type of lithium battery that is particularly useful for 1.5 V consumer devices is the lithium-iron disulfide (or LiFeS2) battery, having the IEC designations FR6 for AA size and FR03 for AAA size. LiFeS2 cells offer higher energy density, especially at high drain rates in comparison to alkaline, carbon zinc or other primary (i.e., non-rechargeable) battery systems. Such batteries use iron disulfide, FeS2 (also referred to as pyrite or iron pyrite, which the preferred mineral form of iron disulfide for battery applications), as the electrochemically active material of the positive electrode.
As a general rule, the electrolyte in any battery must be selected to provide sufficient electrical conductivity to meet the cell discharge requirements over the desired temperature range. As demonstrated by U.S. Pat. No. 4,129,691 to Broussely, increasing the solute (i.e., salt) concentration in a lithium battery electrolyte is expected to result in a corresponding increase in the conductivity and usefulness of that electrolyte. However, other limitations—such as the solubility of the solute in specific solvents, the formation of an appropriate passivating layer on lithium-based electrodes and/or the compatibility of the solvent with the electrochemically active or other materials in the cell—make the selection of an appropriate electrolyte system difficult. As a non-limiting example, U.S. Pat. No. 4,804,595 to Bakos describes how certain ethers are not miscible with solvents such as propylene carbonate. Additional electrolyte deficiencies and incompatibilities are well known and documented in this art, particularly as they relate to LiFeS2 cells and lithium's reactivity with many liquids, solvents and common polymeric sealing materials.
Ethers are often desirable as lithium battery electrolyte solvents because of their generally low viscosity, good wetting capability, good low temperature discharge performance and good high rate discharge performance, although their polarity is relatively low compared to some other common solvents. Ethers are particularly useful in cells with pyrite because they tend to be more stable as compared to higher voltage cathode materials in ethers, where degradation of the electrode surface or unwanted reactions with the solvent(s) might occur (e.g., polymerization). Among the ethers that have been used in LiFeS2 cells are 1,2-dimethoxyethane (“DME”) and 1,3-dioxolane (“DIOX”), whether used together as taught by U.S. Pat. No. 5,514,491 or 6,218,054 or European Patent 0 529 802 B1, all to Webber, or used in whole or in part as a blend of solvents as suggested by U.S. Pat. No. 7,316,868 to Gorkovenko (use of DIOX and 5-6 carbon 1,3-dialkoxyalkanes); U.S. Pat. No. 3,996,069 to Kronenberg (use of 3-methyl-2-oxazolidone and DIOX and/or DME); or U.S. Patent Publication No. 2008/0026296A1 to Bowden (use of sulfolane and DME).
Other solvents not specifically containing DIOX or DME may also be possible, such as those disclosed in U.S. Pat. No. 5,229,227 to Webber (use of 3-methyl-2-oxazolidone with polyalkylyene glycol ethers such as diglyme). However, because of interactions among solvents, as well as the potential effects of solutes and/or electrode materials on those solvents, ideal electrolyte solvent blends and the resulting discharge performance of the cell are often difficult to predict without actually testing the proposed blend in a functioning electrochemical cell.
Another class of ethers has been proposed for use as electrolytes, as disclosed in U.S. Pat. No. 7,316,868. DIOX is used in the blend but the DME is preferentially replaced by one or more 1,2- or 1,3-dialkoxyalkanes having 5 or 6 carbon atoms, such as 1-ethoxy-2-methoxyethane (“EME”), 1-methoxy-2-propoxyethane, 1,2-dimethoxypropane, 1-ethoxy-2-methoxypropane, 2-ethoxy-1-methoxypropane, 1,3-dimethoxypropane, and 1,3-dimethoxybutane. The resulting solvent blend is expected to have particular utility in enhancing the cycle life of lithium-sulfur batteries specifically in comparison to previously known electrolytes containing DME instead of EME (see, e.g., Table 2).
A wide variety of solutes has been used in LiFeS2 cell electrolytes, including lithium iodide (LiI), lithium trifluoromethanesulfonate (LiCF3SO3 or “lithium triflate”), lithium bistrifluoromethylsulfonyl imide (Li(CF3SO2)2N or “lithium imide”), lithium perchlorate (LiClO4), lithium hexafluoroarsenate (LiAsF6) and others. While electrolytes containing lithium triflate can provide fair cell electrical and discharge characteristics, such electrolytes have relatively low electrical conductivity. Furthermore, lithium triflate is relatively expensive. Lithium iodide (LiI) has been used as an alternative to lithium triflate to both reduce cost and improve cell electrical performance, as discussed in the previously identified U.S. Pat. No. 5,514,491 to Webber. One particular brand of AA−-sized FROG batteries sold by Energizer Holdings Inc. currently uses a nonaqueous electrolyte with 0.75 molar concentration of LiI salt in a solvent mixture containing DIOX and DME.
Lithium iodide and lithium triflate salts have been used in combination to provide improved low temperature discharge performance, as described in related U.S. Patent Publication No. 2006/0046154 to Webber. As discussed therein, LiFeS2 cells with a high ether content and LiI as a solute (either the sole solute or in combination with lithium triflate) may sometimes, on high rate discharge at low temperatures, exhibit a rapid drop in voltage at the beginning of discharge. The voltage can drop so low that a device being powered by the cell will not operate. Eliminating LiI as a solute and making lithium triflate the sole solute can solve this problem, but the operating voltage can then be too low on high rate and high power discharge at room temperature. And the use of perchlorates as the sole, primary salt or even as a co-salt may be problematic because of the potential health and safety issues posed by these compounds.
Additives may be employed in the electrolyte to enhance certain aspects of a cell and/or its performance. For example, U.S. Pat. No. 5,691,083 to Bolster describes the use of a very low concentration of potassium salt additives to achieve a desired open circuit voltage in cells with a cathode material including FeS2, MnO2 or TiS2. U.S. Publication No. 2008/0026290 to Jiang discloses the use of an aluminum additive to slow the development of a passivation film on the surface of the lithium electrode. In each of these examples, the benefit of the additive(s) selected must be balanced against any deleterious reactions or effects (in terms of discharge performance, safety and longevity of the battery).
Finally, as mentioned above, it is believed higher concentrations of solute(s) normally improve the conductivity of the electrolyte. However, certain systems (typically in rechargeable lithium-sulfur battery systems where non-chalcogenic polysulfides are the preferred cathode material) utilize a “catholyte” where portions of the electrode itself dissolve into the electrolyte solution to provide ionic conductivity. In such systems, minimal to non-existent concentrations of lithium ions may be provided to a fully charged cell without compromising performance as taught by U.S. Pat. No. 7,189,477 to Mikhaylik. Ultimately, LiFeS2 and other lithium electrochemical cells do not exhibit this propensity to provide ions from the electrodes to the electrolyte, thereby eliminating the usefulness of this approach in LiFeS2 systems and more generally illustrating the pitfalls associated with blindly applying teachings from a given electrochemical system to another, dissimilar system.